Don't measure rotation based on whether your hand is palm down, as your hand can rotate further after the wrist has stopped. Instead, use the wrist bones as your guide. I like to use a ruler that sits across both wrist bones to help me see what's going on. If you see the ruler isn't horizontal after testing, then the rotator muscles are likely too short and forearm rotation has been compromised. Also upper arms and/or shoulders that aren't properly positioned or stabilized can give a false result.
Next, rotate your hands so your palms are facing up (supinated). Again, check your wrist's angle, not your palm's angle, using the ruler as your guide. Did your elbows slide toward your trunk while testing this motion? If so, retest with your arm bone and shoulder blade stabilized.
How did you do? Did you find that you had to work hard to control your shoulder blade and elbow while testing? If so, chances are your forearm's rotator muscles are tight and need to be stretched, even if your forearm was able to rotate fully.
Find out more about this book:Fixing You: Shoulder & Elbow Pain: Self-treatment for rotator cuff strain, shoulder impingement, tennis elbow, golfer's elbow, and other diagnoses.